The present invention relates to the field of geophones in marine seismic operations. More particularly, the invention relates to an improved geophone housing and method for increasing the coupling between geophones and the seafloor. Improved coupling increases the fidelity of the full wavefield recorded by a sensor.
Ocean bottom cables having multi-component geophone sensors detect energy reflected from subsurface geologic formations. Coupling between geophone sensors and the seafloor is essential to the gathering and processing of accurate seismic data. In conventional operations, geophone sensors were taped to the cable exterior before the cable was laid on the seafloor. Geophone quality for such systems fluctuated depending on the effectiveness of the coupling between the sensor and seafloor. When geophone sensors are taped to a bottom cable, clocking problems arise and the sensor coupling to the seafloor is not efficient because sensor orientation is difficult to control. As the cable is laid from the water surface to the seafloor, the taped sensors can land on top, below, or beside the cable. This variation causes poor sensor/seafloor coupling, resulting in variable inline and crossline transfer functions which cause frequency variations between inline and crossline geophones in the cable.
Certain seismic cable systems have been developed to pull cable over large distances so that hand labor to reset each geophone is reduced. U.S. Pat. No. 3,825,886 to Thigpen (1974) disclosed a land based towable seismic detector casing having a flat cable between adjacent detector casings. U.S. Pat. No. 3,954,154 Kruppenback et al. (1976) disclosed another system for a towed land cable. Similar actices exist for ocean bottom and bottom drag cables as the vessel pulls the cable from one position to another.
Various systems have been proposed to enhance sensor coupling with the seafloor. U.S. Pat. No. 5,142,499 to Fletcher (1992) disclosed a marine seismic receiver array having a pointed end for engaging the seafloor. U.S. Pat. No. 5,253,223 to Svenning et al. (1993) disclosed a marine geophone housing having a pointed end for anchoring the housing to the seafloor.
Housings to contain geophone sensors have been developed. U.S. Pat. No. 4,870,625 to Young (1989) disclosed housings for enclosing geophone sensors. Hydrophone sensors were suspended above the seafloor while the geophone sensors were encapsulated in housings laid on the seafloor. The housings included serrations for resisting lateral movement of the housings due to ocean currents. The serrations were specifically designed to resist movement caused by ordinary wave action and to avoid burrowing into the seafloor on the grounds that such burrowing would complicate removal of the geophones from contact with the seafloor. The detached nature and shape of the device potentially improved its coupling ability while significantly complicating retrieval and deployment of the sensors and cabling system. U.S. Pat. No. 5,365,491 to Sullivan et al. (1994) disclosed a marine geophone anchor having pockets for containing a weighted material such as sand.
Seafloor soil conditions can vary significantly from one survey site to another, and within the same site. Sensor housings useful in one soil condition may not adequately provide effective seafloor coupling along the entire survey area. Accordingly, a need exists for an improved apparatus and method for containing geophone sensors, and for coupling such sensors to the seafloor.